In the comics, Rick isn't immune. The story progresses with him facing various challenges and threats like any other character. His survival depends on his wits and the support of the group around him.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
One implication is that he becomes a more formidable opponent in battles. Enemies who rely on poison - tipped weapons or poison - based jutsus would be at a disadvantage against him.